1. Field of the Invention
The present invention relates to a heat treatment apparatus and method for heating a semiconductor wafer, a glass substrate for liquid crystal display, and the like (hereinafter referred to simply as a “substrate”) by light irradiation of the substrate.
2. Description of the Background Art
Conventionally, a lamp annealer employing halogen lamps has been commonly used in the step of activating ions in a semiconductor wafer after ion implantation. Such a lamp annealer carries out the activation of ions in a semiconductor wafer by heating (or annealing) the semiconductor wafer to a temperature of the order of, for example, 1000 to 1100° C. In such a heat treatment apparatus, the energy of light emitted from halogen lamps is used to raise the substrate temperature at a rate of about several hundred degrees per second.
In recent years, with increasing degree of integration of semiconductor devices, it has been desired that the junction be made shallower with decreasing gate length. It has, however, turned out that even if the above lamp annealer, which raises the temperature of a semiconductor wafer at a rate of about several hundred degrees per second, is used to carry out the activation of ions in a semiconductor wafer, there still occurs a phenomenon that boron, phosphorous, or other ions implanted in the semiconductor wafer are deeply heat-diffused. The occurrence of such a phenomenon gives rise to apprehension that the junction may become deeper than the desired level, hindering good device formation.
With regard to this, U.S. Pat. Nos. 6,998,580 and 6,936,797 disclose techniques for raising only the surface temperature of an ion-implanted semiconductor wafer within an extremely short period of time (several milliseconds or less) by irradiating the surface of the semiconductor wafer with flash light from xenon flash lamps (The term “flash lamp” as used hereinafter refers to the “xenon flash lamp.”) The xenon flash lamps have a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of the light emitted from xenon flash lamps is shorter than that of the light emitted from conventional halogen lamps, and it almost coincides with a fundamental absorption band of a silicon semiconductor wafer. Thus, when a semiconductor wafer is irradiated with the flash light emitted from xenon flash lamps, the temperature of the semiconductor wafer can be raised rapidly with only a small amount of light transmitted through the semiconductor wafer. It has also turned out that the flash light emitted within an extremely short period of time such as several milliseconds or less allows a selective temperature rise only near the surface of a semiconductor wafer. Such an extremely-short-time temperature rise with xenon flash lamps will allow only the ion activation to be implemented without deep diffusion of ions.
Now, as a result of the high-energy ion implantation prior to the flash heating, a number of defects are introduced into a silicon crystal of a semiconductor wafer. Such defects tend to be introduced to a somewhat greater depth below the ion-implanted layer. For the implementation of flash heating, it is hence desirable that not only the ion activation but also the restoration of introduced defects be carried out together.
However, in extremely-short-time irradiation where the time of light emission from the flash lamps is only about one millisecond, the speed of a temperature rise at the surface of the semiconductor wafer is higher than the speed of heat transmission to the inside of the semiconductor wafer by thermal conductivity of silicon. This enables a temperature rise in the ion-implanted layer, but not to the depth to which defects are introduced. Nevertheless, if extremely high-energy light is emitted from the flash lamps, it would be possible, even by extremely-short-time irradiation for about one millisecond, to raise the temperature at a depth to which defects are introduced and thereby to restore those defects. However, there arises a problem that the surface temperature would rise considerably, giving damage to the semiconductor wafer.
There has also been a suggestion to prolong the time of light irradiation by the flash lamps to about several milliseconds by controlling the coil constant of a power supply circuit for supplying power to the flash lamps. Such prolonging of the irradiation time to about several milliseconds is considered effective in restoring defects introduced during ion implantation, because it allows a temperature rise not only at the surface of the semiconductor wafer but also to a somewhat greater depth inside the semiconductor wafer. However, there is a possibility that prolonging the time of light irradiation by the flash lamps may cause the generation of new crystal defects because of a continuous temperature rise at the surface of a semiconductor wafer.